This application relates generally to optical communications networks, and more specifically to a method and apparatus for reducing the polarization dependent loss from diffraction gratings used in such communications networks.
The Internet and data communications are causing an explosion in the global demand for bandwidth. Fiber optic telecommunications systems are currently deploying a relatively new technology called dense wavelength division multiplexing (DWDM) to expand the capacity of new and existing optical fiber systems to help satisfy this demand. In DWDM, multiple wavelengths of light simultaneously transport information through a single optical fiber. Each wavelength operates as an individual channel carrying a stream of data. The carrying capacity of a fiber is multiplied by the number of DWDM channels used. Today, DWDM systems using up to 80 channels are available from multiple manufacturers, with more promised in the future.
Optical wavelength routing functions often use demultiplexing of a light stream into its many individual wavelengths, which are then optically directed along different paths. Subsequently, different wavelength signals may then be multiplexed into a common pathway. Within such routing devices, the optical signals are routed between the common and individual optical pathways by a combination of dispersion and focusing mechanisms. The focusing mechanism forms discrete images of the common pathway in each wavelength of the different optical signals and the dispersion mechanism relatively displaces the images along a focal line by amounts that vary with the signal wavelength.
Both phased arrays and reflective diffraction gratings may be used to perform the dispersing functions. While phased arrays are adequate when the number of channels carrying different wavelength signals is small, reflective diffraction gratings are generally preferable when large numbers of channels are used. However, reflective diffraction gratings tend to exhibit greater polarization sensitivity and since the polarization of optical signals often fluctuates in optical communication systems, this sensitivity may result in large variations in transmission efficiency. Loss of information is possible unless compensating amplification of the signals is used to maintain adequate signal-to-noise ratios. Although polarization sensitivity may generally be mitigated by increasing the grating pitch of the reflective grating, limitations on the desired wavelength dispersion for signals at optical telecommunication wavelengths preclude an increase in grating pitch sufficient to achieve high diffraction efficiency in all polarization directions.
Suggestions to reduce polarization dependent losses in optical switching systems have included complex polarization splitting and recombination techniques, such as described in WO 98/35251, published Aug. 13, 1998. In the method described therein, an optical beam is separated into distinct subbeams for different polarization states and optically constrained to follow different paths, which ultimately converge so that the subbeams may be recombined. Creating and maintaining separate optical paths requires additional components and increases both the cost and complexity of the devices that use the method. Furthermore, the recombination of the subbeams requires very precise alignment of the optical components to prevent the introduction of spurious distortion resulting from imperfect recombination.
It is thus desirable to provide a method and apparatus that reduces or eliminates polarization dependent loss from diffraction gratings used in optical telecommunications systems without requiring beams with different polarization states to follow different optical paths.
Embodiments of the invention are directed to a wavelength router for receiving, at an input port, light having a plurality of spectral bands and directing some of those spectral bands to various output ports. In one embodiment, the wavelength router includes an optical arrangement configured to provide optical paths for routing the spectral bands between the input and the output ports. A routing mechanism within the wavelength router has at least one dynamically configurable routing element to direct a given spectral band to different output ports, depending on the state of the dynamically configurable element. The wavelength router also includes a polarization-rotation element disposed with respect to the optical arrangement and the routing mechanism to be encountered by each optical path at least twice.
In certain embodiments, the polarization-rotation element is configured to rotate polarization states by approximately 45xc2x0 with respect to fixed orthogonal axes. Thus, embodiments use a quarter-wave plate or Faraday rotator to achieve the polarization rotation. In some embodiments, the polarization-rotation element is configured so that it is encountered by each optical path before and after each optical path encounters the routing mechanism.
In one embodiment, the optical arrangement includes a dispersive element and is configured so that light on each optical path encounters the dispersive element twice. The dispersive element may be a reflective or transmissive grating, or may be a prism, in different embodiments. The optical arrangement may also include lens or reflective surfaces to define the optical paths by collimating and focusing light as it is directed to the routing mechanism and output ports.